Greasing shown between the coils indicates end turn vibration. The relative motion of coils wearing away insulation, followed by a mixing of the insulation particles with entrained oil causes the greasing. 
Stator Winding End Turn Vibration
Problem: Stator winding end-turn vibration can wear away coil insulation, precipitating a ground fault, or it can cause coil strand breakage, resulting in overheating. This type of vibration is caused by double-frequency operating forces that loosen the end turns over time. Top photo shows the tell-tale greasing between coils indicative of end-turn vibration. The relative motion of coils wearing way insulation, plus the mixing of the insulation particles with entrained oil, causes the greasing. In bottom photo, dusting, caused by the relative movement between the stator-coil end turns and the support bracket, shows up as a white powder where the bracket and the end-turn surge ring meet.